Epoxides such as ethylene oxide, propylene oxide, 1,2-butene oxide and the like are useful intermediates for the preparation of a wide variety of products. The oxirane functionality in such compounds is highly reactive and may be ring-opened with any number of nucleophilic reactants. For example, epoxides may be hydrolyzed to yield glycols useful as anti-freeze components, food additives, or reactive monomers for the preparation of condensation polymers such as polyesters.
Polyether polyols generated by the ring-opening polymerization of epoxides are widely utilized as intermediates in the preparation of polyurethane foams, elastomers, sealants, coatings, and the like. The reaction of epoxides with alcohols provides glycol ethers, which may be used as polar solvents in a number of applications.
Many different methods for the preparation of epoxides have been developed. One such method involves the epoxidation of an olefin in a liquid phase reaction using an organic hydroperoxide as the oxidizing agent and certain solubilized transition metal compounds as catalyst. The early work in this field concluded that optimum epoxidation rates and selectivity to epoxide generally are obtained using metallic catalysts which are soluble in an organic reaction medium. For example, U.S. Pat. No. 3,350,422 teaches in Example 6 that while vanadium naphthenate (a soluble catalyst) provided 72% hydroperoxide conversion and 38% selectivity to propylene oxide, vanadium pentoxide (an insoluble species) gave only 34% hydroperoxide conversion and 6% propylene oxide selectivity. Similarly, U.S. Pat. No. 3,351,635 teaches that metals such as molybdenum, tungsten and titanium are most effective as epoxidation catalysts when dissolved in the epoxidation reaction mixture. Poorly soluble species such as molybdenum trioxide thus are initially inactive and only become suitable for use in such application when converted to a soluble active form by reaction with alcohol, glycol, hydroperoxide or the like (see, for example, the discussion in Sheldon, J. Mol. Cat. 7, pp. 107-126 (1980)).
A distinct disadvantage of an epoxidation process which utilizes a soluble metallic compound as catalyst is the difficulty associated with recovering the catalyst for reuse in subsequent runs. When the other components of an epoxidation reaction mixture (typically, epoxide, unreacted olefin, solvent, unreacted hydroperoxide, and the alcohol derived from the reacted hydroperoxide) are relatively volatile, these components may be separated from the soluble non-volatile catalyst by distillation and the catalyst recovered in the form of a bottoms stream. A problem associated with such a method, however, is that the bottoms stream may tend to accumulate certain heavy substances such as acids and polymers which may have a deleterious effect on epoxide selectivity or olefin conversion when the stream is reused. The catalyst may also have a tendency to precipitate from solution if the bottoms stream is overly concentrated; recycle of a relatively large bottoms stream may thus be required, which will detrimentally affect the productivity of the epoxidation process. It would therefore be highly desirable to develop an insoluble (heterogeneous) epoxidation catalyst which has high activity and selectivity and which may be readily recovered in active form from an epoxidation reaction mixture by filtration or similar separation techniques or which may be utilized in the form of a fixed bed or the like.